Process for making continuous magnetite films

ABSTRACT

A continuous magnetite film, suitable for magnetic recording, is prepared by forming a film of amorphous Fe2O3, heating the Fe2O3 until it is completely converted to the alpha crystalline phase, and then reducing it to magnetite.

United States Patent Inventors Richard Lawrence Comstock PROCESS FORMAKING CONTINUOUS MAGNETITE FILMS 10 Claims, No Drawings US. Cl 117/237,117/105.3, 117/12 H, 117/127, 117/235, 23/200, 252/62.56 lnt.Cl. 1101110/02 Field olSearch 117/235,

[56] References Cited UNITED STATES PATENTS 2,978,414 4/1961 Han et al.117/235 FOREIGN PATENTS 1,121,826 7/1968 GreatBritain 626,756 9/1961Canada OTHER REFERENCES Kwiatkowski, Chemical Abstracts, Vol. 69, 19681026908 Primary Examiner-William D. Martin Assistant Examiner-Remard DPianalto Attorneys-Hanifin and Jancin and Joseph G. Walsh ABSTRACT: Acontinuous magnetite film, suitable for magnetic recording, is preparedby forming a film of amorphous Fe,0,, heating the Fe,0 until it iscompletely converted to the alpha crystalline phase, and then reducingit to magnetite.

PROCESS FOR MAKING CONTINUOUS MAGNETITE FILMS FIELD OF THE INVENTION Thepresent invention is concerned with the preparation of magnetite films.In particular, it is concerned with the preparation of continuous filmshaving a high magnetic remanence and all other properties making themsuitable for use as magnetic recording media with very high storagedensity.

PRIOR ART Magnetite film recording media are mentioned by Ostertag etal. in IEEE Transactions on Magnetics Vol. Mag. 5, Page 327, Sept. 1969,but no method for their preparation is described, except the statementthat some of them were obtained by controlled oxidation. To the best ofour knowledge, all previous preparations of magnetite films haveinvolved oxidation. A

The kinetics of the reaction between hydrogen and bulk particles of Fetl, is described by van de Giessen et al. in IEEE Transactions onMagnetics Vol. Mag. 5, Page 317, Sept. 1969, but films of alpha phasecrystalline I=e,0 were not used as starting materials and no magnetitefilms were prepared.

It should be noted that alpha phase crystalline Fe,0, is nonmagnetic andthe prior art on the making of magnetic films teaches its avoidance, butit is, surprisingly, as essential intermediate in the present invention.

SUMMARY OF THE INVENTION Magnetic surfaces for magnetic recording athigher density than current practice (2,000 flux changes per inch) mustbe thinner than 2.5 microns and, in contrast to currently usedparticle/binder coatings, they should be magnetically continuous. Theymust also have remanent magnetization in the range 41rM l,000 G, andcoercive field in the range 200 H, l,000 e. For example, to recorddigital bits at 10,000 flux changes per inch (f.c.i.) with remanentmagnetization 41rM,. =4000 G and H =400 Oe, requires a coating thicknessof 0.5 micron. Thinner metal films prepared by electroless deposition,e.g. NiCo-P, may satisfy these requirements, but these metals typicallyhave poor wear characteristics. One solution to the wear problem isovercoating with a hard nonmagnetic metal, e.g. Cr, but this increasesthe critical head/recording surface spacing.

The present invention is a process for making thin (less than 2.5microns, and preferably less than 1 micron) coatings of magnetite (Fe 0which have all of the characteristics necessary for high storage-densitymagnetic recording, including high wear resistance.

There are three steps to the process of the present invention. First, afilm of amorphous Fe,0,is formed. There are several methods which may beused for this step. One preferred method is applying to the substrateferric nitrate solution and spinning on a photoresist spinner. Uponheating, the ferric nitrate decomposes to form a film of amorphousFe,0,.

In the second step of the process, the film of amorphous Fe,0 is heatedabove about 300 C. until it has been completely crystallized to thealpha phase. This crystallization is a critical portion of the process.Only when the alpha crystalline phase is used as the starting materialfor the next step does the final product magnetite have the requiredhigh magnetic remanence. The explanation for this is not known, and theresult was very unexpected.

In the third step of the process, the film of alpha phase Fe,0, isreduced to magnetite, Fe -,0 This reduction may be accomplished in manyways, for example treatment with carbon monoxide or other reducingagents. The preferred method is treated with hydrogen gas, particularlyhydrogen gas containing a small amount of water vapor. A feature of thepresent process which is particularly attractive is the relatively lowtemperature required to form the magnetite. Previously,

when working with the oxidation type reactions, temperatures of 600-l,000 C. were encountered. These high temperatures severely restrict thechoice of substrate to be used because of: (l) mechanical distortions,(2) chemical reactions between film and substrate, (3) recrystallizationand phase changes. The present invention overcomes these problems andhas the advantage of being suitable for use on any of a wide variety ofsubstrates. The substrate should be nonmagnetic and have a smoothsurface. It should be chemically compatible with the film coatings. Itshould resist deformation. Titanium and titanium alloys have beenoutstanding substrates. Good results have been obtained with severalvarieties of glass. Aluminum is an attractive substrate for economicalreasons. Various types of ceramics may also be useful. Alloys, such asnonmagnetic stainless steel, are suitable for use as the substrate.

Mechanical, thermal or chemical processes, or combinations thereof, areperformed to prepare the surface of the chosen substrate for coating.With respect to the coating operation, the main requirement is that thesurface be wetted by the diluted solution for generation of a smoothcontinuous film. Other surface requirements (surface finish, flatness,etc.) are dictated by the end usage. Except for cleaning, glass surfacesare usually ready for use without any other treatment. Metal surfacesare prepared by such methods as lapping, fine grinding with abrasivepaper, metallographic polishing and diamond turning. Magnetic propertiesof the films have been found to be relatively insensitive to the type ofsubstrate and to the surface preparation.

The films produced by the process of the present invention are randomlyoriented, polycrystalline, continuous films of nominal Fed),composition. The grain size has been determined microscopically to be0.15 micron or smaller. Intrinsic surface finish (as obtained on glasssubstrates) is estimated to be about 0.02 micron peak to peak. Thicknessuniformity has been found to be better than 5 percent over a lineardimension of several inches for the spinning technique.

The following examples are given to illustrate the preferred method ofcarrying out the invention. They are for purposes of illustration only,and are not to be deemed limitations of the invention, many variationsof which are possible without departing from the spirit or scopethereof.

EXAMPLE I Prepare a concentrated (l0 molar) ferric nitrate stocksolution using Fe(NO -,);,-9H,0 and water. Dilute one part stocksolution to two parts ethyl alcohol (denatured). Filter as required.(The use of ferric nitrate is a matter of convenience. The same resultcan be obtained by dissolving Fe, Fe,0,, etc. in nitric acid. Also, theconcentration of the stock solution may be reduced as desired. A 10molar solution yields about 0.1 micron per coat).

The substrate is held by a suitable rotating device and the dilutedsolution applied. A wet film is formed during rotation and is stabilizedby evaporation of most of the alcohol. A very good practice is to use aphotoresist spinner, apply two to 10 drops of solution (depending onsize of substrate) to the center of the substrate, start spinner andspin for l5 seconds at 2,400-5,000 r.p.m. (depending on size ofsubstrate).

Should visual observation reveal defects (particles, bubbles, etc.), thewet coating may be removed at this point with a solvent (alcohol,acetone, water, etc.) and a fresh coating applied.

Drying is achieved by heating the coating substrate in air to a suitabletemperature. Our present practice employs a hot plate which is runthrough a timed cycle achieving 450-500 C. maximum. As the film isheated, the last of the alcohol and the water is driven off. Furtherheating decomposes the nitrate, giving ofi nitric and nitrous oxides andleaving an amorphous solid with the composition Fe ll At temperatures ofabout 300 C. crystallization of alpha Fed), begins and heating iscontinued to completely crystallize the film.

Additional coatings may now be applied, if desired, to build upthickness, since the solid Fe,0 is relatively insoluble in the spinningsolution.

The alpha Fe,0 is reduced to magnetic Pe o at elevated EXAMPLE lllMagnetic Recording 3-inch disks of magnetite on the titanium alloysubstrate (90 percent Ti, 6 percent al., 4 percent V) have been preparedtemperature in a suitable atmosphere. A very good practice 5 with anapproximate Q7 micron AA smoothness with employs hydrogen gas andsdescribed by the reaction: recordings made under the same conditions, wehave 3F +H 2F 0 0 discovered that the magnetite films result in a 100percent ino a crease in readback signal as compared with a commerciallyThe iemperamie range used 350 i f used 'yFe o lbinder coating which istwice as thick. This 100 perature range, complete reduction to metalliciron rs likely, 10 percent improvement was observed at recordingdensities and so to prevent thrs the hydrogen gas I bubbled throughgreater than 2,000 flux changes per inch. At the recorded denwatei' suchthat the P z P z is approximately 0-025 iii sity at which the magnetitesurface gave 100 percent larger the furnace atmosphei'e- The time atieiiipei'aiui'e i5 amplitude than the conventional surface the peakshift of the y diffi'aciioii aiiaiysis ofa yp iiim detected y recordedflux change was less than i percent, compared to 5 magnetite. PC3041after this operationpercent for the commercial surface.

A time/temperature study of Fe t). film formation using Read/write testson 3- inch disks are summarized in the folhydrogen gas mixed with watervapor has been completed. lowing table:

Read-back amplitude Roman- (relative) ence Filmthickness,microns 41rM.,g. ratio 41rl\ g. li en. 1,240 120.1. 4,000 l.c.i.

With a room temperature dew point the degree of reduction of 2Fe, 0 toFe t) is strongly dependent on temperature in the range of 325-400 C.The optimum was found to be between 350375C. for 1 hour.

Two styles of furnaces have been used for this operation. A 2-inch tubefurnace has been used for small test pieces, while the 3-inch disks havebeen treated in a box furnace with an inconel muffle. Both furnacesprovide positive control of gas purity and easy disposal of theflammable hydrogen.

EXAMPLE ll The following table shows physical properties of filmsprepared by the process of the present invention.

Film Saturation Remanent thiekmagnetiza- Coercive n ss, tionMagnotizafield, Substrate microns (41rMs), E. Ratio tion, g. 00.

Ti (6 Al, 4 V)... 0. 66 844 0. 59 497 240 A1 0. 66 3, 860 0. 72 8, 770280 Stainless steel... 0. 66 4, 200 0. 74 3, 100 380 T1 (6 A1, 4 V)..-0. 44 5, 000 0. 74 3, 700 510 Glass 0.44 2, 400 0. 71 1, 700 560 0. 444, 700 0. 74 3, 480 655 0. 70 4,600 0. 70 3, 220 700 0.44 3, 600 0. 78,360 965 0. 66 6, 000 0. 74 4, 400 310 0. 44 4, 360 0. 81 3, 500 465What is claimed is:

l. A process for making a continuous thin film having a high magneticremanence and suitable for use as a high storagedensity magneticrecording medium, said process comprising:

a. forming a film of amorphous Fe,0 of less than 2.5

microns thickness;

b. heating the film of amorphous Fe,0, above about 300 C. until it iscompletely converted to the alpha crystalline phase, and

c. reducing the alpha phase Fe,0 to magnetite.

2. A process as claimed in claim 1 wherein the amorphous Fe,0 film isless than 1 micron thick.

3. A process as claimed in claim 1 wherein the amorphous Fe,0 film isformed by a spinning technique.

4. A process as claimed in claim 1 wherein the reduction is carried outby reaction with hydrogen gas.

5. A process as claimed in claim 4 wherein the temperature duringreduction is between 325 and 400 C.

6. A process as claimed in claim 4 wherein the hydrogen gas contains asmall amount of water vapor.

7. A process as claimed in claim 1 wherein the film is deposited upon anonmagnetic metallic substrate.

8. A process as claimed in claim 1 wherein the film is deposited upon atitanium substrate.

9. A process as claimed in claim l wherein the film is deposited upon analuminum substrate.

10. A process as claimed in claim I wherein the film is deposited upon aglass substrate.

2. A process as claimed in claim 1 wherein the amorphous Fe203 film isless than 1 micron thick.
 3. A process as claimed in claim 1 wherein theamorphous Fe203 film is formed by a spinning technique.
 4. A process asclaimed in claim 1 wherein the reduction is carried out by reaction withhydrogen gas.
 5. A process as claimed in claim 4 wherein the temperatureduring reduction is between 325 and 400* C.
 6. A process as claimed inclaim 4 wherein the hydrogen gas contains a small amount of water vapor.7. A process as claimed in claim 1 wherein the film is deposited upon anonmagnetic metallic substrate.
 8. A process as claimed in claim 1wherein the film is deposited upon a titanium substrate.
 9. A process asclaimed in claim 1 wherein the film is deposited upon an aluminumsubstrate.
 10. A process as claimed in claim 1 wherein the film isdeposited upon a glass substrate.